I did a little exploring on some different types of cameras and their storage time.
For Ring cameras, the default for Ring Protect subscribers is 60 days. Subscribers can adjust from it from 1 day up to 180 days. If you don't subscribe to Ring Protect, it's live only.
Learn how to adjust how long your recorded videos are stored with a Ring Protect Plan or Trial.
ring.com
At intersections, many cams only record if speeding, running a red light, or an accident is detected.
Other DOT cams are live-stream only, for maintenance purposes.
Based on the sources I could find online, most traffic cams that do record keep the footage for a limited time, but this varies greatly from state to state. New York is apparently only 30 days, California is five years. Many states appear to be in the 30-90 day range.
I don't know how long it is in Idaho, the following article is all I could find:
cdapress.com
Apparently, Washington State does not retain video footage at all.
"We do not retain or record video footage of the roadway from our network of intersection and freeway cameras. Unfortunately, this means we cannot provide videos on specific dates or times for traffic collisions or other events."
See here:
Request and view our collection of digital imagery of the state highway system, collected biyearly. Use our state route viewer tool to view the roadway by milepost.
wsdot.wa.gov
If these last two are true, it would appear most video footage would have come from private Ring or other security cams, owned by individuals and businesses, including WSU and UofI, rather than DOT.
Can anyone local confirm?